The present invention relates to an industrial vehicle that includes a hydraulic mechanism and a method for controlling the industrial vehicle.
One example of an industrial vehicle including a hydraulic mechanism is an engine forklift used to lift and move materials. Such a forklift uses the hydraulic mechanism to lift and lower (lift operation) and tilt forward and rearward (tilt operation) forks located at the front of the forklift when handling materials. The hydraulic mechanism uses a hydraulic pump driven by the engine of the forklift when performing a lift operation and a tilt operation.
Regulations that restrict exhaust gas emissions have become strict and are also applied to industrial vehicles. This has resulted in a need to improve fuel efficiency. To do so, the speed and output of the engine may be reduced when the engine is idling and the forklift is not handling materials. When the engine is idling, the operation of the hydraulic mechanism to move the forks and handle a material produces a load that further reduces the speed and output of the engine. This may destabilize the engine behavior and cause knocking, deteriorate the exhaust gas components, or stall the engine.
Japanese Laid-Open Patent Publication No. 2012-62137 describes an example of a conventional hydraulic system for an industrial vehicle. The hydraulic system includes a material handling pump, which is driven by an engine and which discharges hydraulic oil, a material handling hydraulic actuator, and a control valve, which controls the supply of hydraulic oil to the hydraulic actuator. A hydraulic pipe connects the control valve to the pump. A discharge pipe, branched from the hydraulic pipe, is connected to a hydraulic oil tank. An unload valve is arranged in the discharge pipe to open and close the discharge pipe. In a normal material handling operation, the unload valve closes the discharge pipe. Thus, the hydraulic oil from the pump is sent to the control valve. When the engine speed is less than or equal to a predetermined speed or the pressure of the hydraulic oil in the hydraulic pipe is greater than or equal to a predetermined value, a controller, which controls the unload valve, determines that the engine may stall and thus operates the unload valve to open the discharge pipe. This reduces the load on the pump and prevents engine stalling.
Further, the '137 publication describes a pump load reduction system for the industrial vehicle. The system is arranged in the hydraulic pipe that connects the material handling pump and the control valve. The pump load reduction system includes a switch valve, which is arranged in the discharge pipe, a sub-relief valve, which is arranged in the discharge pipe at the downstream side of the switching valve, and a check valve, which is arranged in the hydraulic pipe at the downstream side of where the discharge pipe is branched. When the engine speed is high, the switch valve is controlled to send the hydraulic oil to the downstream control valve in the hydraulic pipe. When the engine speed is lower than the predetermined value, the switch valve is controlled to send the hydraulic oil to the discharge pipe. In the pump load reduction system, when the hydraulic pressure of the hydraulic oil in the pipes reaches the pressure that opens the sub-relief valve, the sub-relief valve opens and discharges the hydraulic oil through the discharge pipe. This keeps the hydraulic pressure in the pipe less than the pressure that opens the sub-relief valve. Accordingly, the pump load reduction system reduces the load on the pump and prevents engine stalling.
However, in the hydraulic system of the '137 publication, during a material handling operation, the opening of the unload valve and the switching of the switch valve is performed after detection of a decrease in the engine speed to the predetermined speed or less. Thus, when load is applied to the pump and the engine, the engine speed may be temporarily reduced to less than the predetermined speed. In this case, when the predetermined speed is set to the idling speed, the engine speed may become less than the idling speed. This may destabilize the engine behavior and cause knocking, deteriorate the exhaust gas components, or stall the engine.
Further, in the pump load reduction system of the '137 publication, load is also applied to the pump and the engine during a material handling operation. Thus, the engine speed may become less than the idling speed.